Aircraft engine NOX emissions

Turbojet and turbofan engines

Engine technology has continuously evolved over the last 70 years, and reduction in fuel burn has always been a driving force behind this progress. More fuel efficient engine cycles, often made possible through the use of new materials, has led to increasing pressures and temperature within the combustor. Since this tends to increase the emissions of nitrogen oxides (NOX), the control of these emissions through the combustor design is a significant challenge. The ICAO regulatory limits for engine NOX emissions has been gradually tightened over time, and are usually referred to by the corresponding CAEP meeting number (CAEP/2, CAEP/4, CAEP/6 and CAEP/8). The engine NOX standard, and the new aeroplane CO2 standard, contribute in defining the design space for new products so as to address both air quality and climate change issues.

Figure 2.5 illustrates certified NOX emissions data of aircraft engine models above 89 kN thrust in relation to the ICAO CAEP NOX limits [25]. The regulatory NOX limits are defined as the mass (Dp) of NOX emitted during the Landing and Take- Off (LTO) test cycle and divided by the thrust of the engine (F00). The limit also depends on the overall pressure ratio7 of the engine. The current ICAO technology goals for NOX are also shown. These goals, which were agreed in 2007, represent the expected performance of expected ‘leading edge’ technology in 2016 (mid-term) and 2026 (long term).

Each point in Figure 2.5 represents EASA certified data for an engine model, and the different colours provide insight into the trend over time. The dataset represents engine models typically fitted to single-aisle aircraft (e.g. A320, B737) and larger aircraft (e.g. A350, B777, A380). No further versions of the leading edge GEnx engines (lower green dots) have been certified since 2015. However, the most recent data (purple diamonds) illustrate that other manufacturers on different product development cycles have optimised new and existing combustor designs.


7 Ratio of total pressure at compressor exit compared to pressure at engine inlet.